In recent studies on catalysts, those in which a catalyst metal for catalysis alone is supported, and those in which a metal in addition to a catalyst metal is supported in combination for improving activity, improving properties such as prevention of grain growth and prevention of catalyst poisoning, or for imparting additional functions (hereinafter metals supported for improving properties or imparting functions are referred to as a promoter metal) have been developed in many ways. Examples of these include a platinum/rhodium/barium composite catalyst known as a catalyst for purification of car exhaust, which has an NOx adsorption ability.
In addition to the conventional role in combustion of exhaust gas, platinum/rhodium/barium composite catalysts have been developed as a catalyst which can efficiently remove NOx. In these composite catalysts, platinum/rhodium facilitate usual combustion of exhaust gas, while barium, which is a promoter metal, is responsible for NOx adsorption.
It is considered that in such composite, NOx adsorption action according to barium takes place by way of precious metal. The NOx adsorption ability of this composite catalyst is considered to be influenced by the distance between the precious metal and barium supported on the support, and how close they are supported is very important.
For producing a platinum/rhodium/barium composite catalyst, the present inventors have disclosed a catalyst using colloid in the production steps, aiming at improving dispersibility of barium and the supporting state (Japanese Patent Application Laid-open No. 2000-279818 and Japanese Patent Application Laid-open No. 2000-279824). In this case, the colloid means those in which fine particles (cluster particles) of metal or ceramics insoluble in solvent are dispersed or suspended in the solvent, and colloid solutions using liquid as a solvent are generally known. By allowing such colloid to be adsorbed on a support, metal fine particles forming cluster particles can be directly dispersed and supported on a support to a high degree unlike the usual cases of using an aqueous solution.
Referring to these conventional arts, in Japanese Patent Application Laid-open No. 2000-279818, single metal colloid or composite metal colloid comprising one or more kinds of metal particles (e.g., platinum/rhodium bimetallic colloid) is supported on a support which is a porous material, and alkali metal, alkaline-earth metal (e.g., barium) or rare earth metal is allowed to be supported thereon using an aqueous solution of any compound of alkali metal, alkaline-earth metal or rare earth metal.
In addition, Japanese Patent Application Laid-open No. 2000-279824 discloses employing a given polymer organic compound as a protective agent constituting colloid. This protective agent is a compound chemically or physically bonded to or adsorbed on colloid particles in a colloid solution, and prevents colloid particles from agglomerating, thereby controlling the particle distribution to be settled in an appropriate range. In other words, by adding a protective agent, a suspended state of colloid particles of a small particle size can be maintained and when a catalyst is produced, the particle size of catalyst particles can be made smaller and the effective surface area of the catalyst can be increased to the maximum. Although the protective agent is referred to as a chelating agent in this conventional art, both have the same meaning.
In the above-mentioned conventional arts, dispersibility of barium is certainly improved, but they are not necessarily sufficient in that barium and precious metal are to be supported in close vicinity with each other. The present inventors have then confirmed that in the case of catalysts produced according to such conventional arts, all barium actually supported cannot be fully utilized, and the actual utilization ratio thereof is only about a dozen % by mol or so.
The problem as described above is not limited to platinum/rhodium/barium composite catalysts. Other than these exhaust gas purification catalysts, for example, catalysts in which a promoter metal is supported in combination in addition to a catalyst metal are also being studied as catalysts for fuel cells in many cases. In most of these catalysts, it is preferable that the catalyst metal and the promoter metal are supported in close vicinity with each other.
The present invention has been made in view of such circumstances, and aims at providing a catalyst in which a catalyst metal and a promoter metal are supported in combination on a support, wherein they are supported in such a close distance that the characteristics of both metals are sufficiently exhibited.